1. Field of the Invention
The present invention relates to an optical disc apparatus controlling tracking by the sampling servo system.
2. Description of the Related Art
In the art of the system in which an optical recording medium is used as the recording medium, such as, for example, a magneto-optical disk apparatus, an optical disc apparatus etc. (to be generically termed as optical disc apparatus hereinafter), there are provided some kind of tracking systems for trailing tracks formed on an optical recording medium by the laser beam of an optical pick-up device for recording data on, and/or regenerative data from, an optical recording medium. One of the systems is constructed by a continuous servo method in which a guide slot for the laser beam is formed, whereas as another system, there is a sampling servo method in which servo patterns are recorded on the mirror-finished recording medium for the purpose of detecting the positional errors of the laser beam. In addition, some variations of these servo methods are being brought into practice.
As an reference example of the sampling servo system, a disclosure has been made in Japanese Patent Application Laying-open No. 243241/1989. In this conventional system, there are provided on each servo field of an optical recording medium a wobble pit pair for the detection of the tracking error signal, a clock pit for the detection of timing and address pit section for detecting the address of a recording track. One pattern of the address pit section has a 12-digit length and there are 28 patterns, which are allotted respectively to different individual tracks, cyclically being repeated by the same pattern after every 28 tracks.
Thus, the patterns of address pit section are cyclically repeated in every 28 tracks, so that in finding or identifying the destination address of a recording track by referring these patterns for an address pit section, the optical pick-up can move to a following servo field to be read out on any recording track located within 27 tracks from the track before the movement.
Accordingly, it is possible for the optical pick-up of the system of this example to move from the starting recording track to a recording track located within 27 tracks apart, with the time taken for the optical disc to be turned from one servo field to a next servo field. As a result, the optical pick-up can be moved at a high speed, thus making it possible to shorten the time for a data access.
However, thus configurated system has drawbacks as follows.
Specifically, when the travel speed of the optical pick-up device is intended to improve in order to shorten the time taken for a data access, this requires increase in the number of the patterns for the address pit section. To achieve this, it is necessary to increase in the number of the digits of the address pits in a pattern, so that the data length in the servo field becomes long, resulting disadvantageously in a reduction of the data storing capacity for one optical disc.
On the other hand, when the number of digits of an address pit sectional pattern in the servo filed is to be reduced in order to increase the data storing capacity for one optical disc, the kinds of the patterns for the address pit section are decreased. Consequently, as a result, the travel speed of the optical pick-up is required to be low, thus giving rise to an inconvenience that the time taken for a data access becomes lingered.